Two potent angiogenesis inhibitors, angiostatin (O'Reilly et al. (1994) Cell 79:315) and endostatin (O'Reilly et al. (1997) Cell 88:277), were discovered and found to be generated naturally by primary tumors. Both proteins are specific inhibitors of endothelial cell proliferation and inhibit tumor growth by blocking angiogenesis, the formation of new blood vessels that nourish tumors. Studies have shown that these angiogenesis inhibitors are non-toxic even at very high doses and that they may suppressed the growth of metastases and primary tumors may regress to a dormant microscopic state. Both inhibitors were identified as proteolytic fragments of much larger intact molecules. Angiostatin was found to be a fragment of plasminogen, and endostatin a fragment of collagen XVIII.
These two proteins have generated great interest in the cancer area because they have been shown to suppress the growth of many different types of tumors in mice, with no obvious side effects or drug resistance. Traditional chemotherapy generally leads to acquired drug resistance caused primarily by the genetic instability of cancer cells. Rather than targeting cancer cells, therapies using angiogenesis inhibitors target the normal endothelial cells, which support the growth of the tumor. Because endothelial cells are genetically stable, it is possible that angiogenesis inhibitor therapies may result in less drug resistance. Studies indicate that drug resistance did not develop in mice exposed to prolonged anti-angiogenic therapy using endostatin. Furthermore, repeated cycles of endostatin treatment in mice resulted in prolonged tumor dormancy and no recurrence of tumors following discontinuation of therapy (Boehm et al. (1997) Nature 390:404).
Despite promising results in mice, it has not been possible to produce clinical grade soluble, active angiostatin and endostatin in commercial quantities using E. coli, baculoviral, yeast, and mammalian expression systems. Expression in E. coli yielded insoluble protein aggregates of undefined composition, which could not be injected into humans. Other production methods, such as baculovirus and mammalian expression systems, yielded very low levels of the recombinant proteins (O'Reilly et al. (1997) Cell 88:277).
The poor yields of the expression systems to date may be explained by both angiostatin and endostatin being internal fragments of much larger proteins. The truncated proteins may not fold properly in the absence of the residues that are cleaved from the precursor molecules. For example, angiostatin has 26 cysteine residues which form numerous disulfide bonds. Expression of angiostatin by itself may not provide the optimal environment for these numerous disulfide bonds to form correctly in the secretory pathway. Also, the recombinant endostatin protein produced in E. coli precipitated during dialysis, possibly due to the hydrophobicity of endostatin (O'Reilly et al. (1997) Cell 88:277).
A major hurdle with the use of angiostatin and endostatin in their present forms is that relatively large amounts of proteins have to be injected daily for weeks to months to achieve the desired clinical outcome. For example, in current mouse models, dosages of 20 mg/kg/day of endostatin are needed to demonstrate optimal efficacy (Boehm et al. (1997) Nature 390:404). Given that there is an urgent need to test endostatin and angiostatin clinically, a production method that can generate large quantities of clinical grade material is important.
One expression system that has been used to produce high level expression of fusion proteins in mammalian cells is a DNA construct encoding, a signal sequence, an immunoglobulin Fc region and a target protein. The fusion product of this construct generally is termed an “immunofusin.” Several target proteins have been expressed successfully as immunofusins which include: IL2, CD26, Tat, Rev, OSF-2, βIG-H3, IgE Receptor, PSMA, and gp120. These expression constructs are disclosed in U.S. Pat. No. 5,541,087 and U.S. Pat. No. 5,726,044, the disclosures of which are incorporated herein by reference.
A major purpose of expressing recombinant fusion proteins in mammalian cells has been to attempt to confer novel or useful properties to the hybrid molecules, e.g., proper folding, increased solubility, targeting of a cytokine or toxin in vivo, Fc receptor binding, complement fixation, protein A binding, increased circulation half-life, and increased ability to cross the blood-brain barrier. Examples of recombinant fusion proteins produced in mammalian cells include cytokine immunoconjugates (Gillies et al. (1992) Proc. Natl. Acad. Sci. USA 89:1428; Gillies et al. (1993) Bioconjugate Chemistry 4:230), immunoadhesins (Capon et al. (1989) Nature 337:525), immunotoxins (Chaudhary et al. (1989) Nature 339:394), and a nerve growth factor conjugate (Friden et al. (1993) Science 259:373). Each of the foregoing publications is incorporated herein by reference.
It is an object of the invention to provide novel DNAs which facilitate efficient production and secretion of angiogenesis inhibitors in a variety of mammalian host cells. It is another object of the invention to provide methods for treating mammals with nucleic acids encoding, or amino acid sequences defining angiogenesis inhibitor proteins, including non-native, biosynthetic, or otherwise artificial proteins such as proteins which have been created by rational design.